The present exemplary embodiment relates to document processing systems. It finds particular application in conjunction with background suppression in color scanning and copying systems and will be described with a particular reference thereto. However, it is to be appreciated that the present exemplary embodiment is also amenable to other document processing systems and like applications.
Typically, in a digital reproduction device, a document or image is scanned by a digital scanner which converts the light reflected from the document into electrical charges representing the light intensity from predetermined areas (pixels) of the document. The pixels of image data are processed by an image processing system which converts the pixels of image data into signals which can be utilized by the digital reproduction machine to recreate the scanned image.
Sometimes the white regions in the original are not imaged as white regions in the produced document. Typically, digital scanning and multifunction devices provide the function of automatic background suppression to make white regions in the original white on scanned images or copies. The quality of background suppression is of great importance to many customers. Background detection and suppression in color systems is more challenging than in monochrome systems due to the more complex nature of the problem.
One method of color background detection/suppression is to perform the noise suppression in each of the RGB channels separately. Initially, the background or white pixel value is determined by any of known methods as, for example, described in U.S. Pat. No. 6,674,899 to Nagarajan, identified below. The background pixel values of each channel are detected separately and mapped to 255 in each individual RGB channel. However, this method can produce artifacts in the resultant image as each channel's value of white pixel may vary. For example, the white pixel value in the blue channel may be 250, while the white pixel value in the green channel may be 235. By mapping the background pixels in each channel to 255, the color in the resultant image may be changed. Such technique could be expensive as the processing of three channels is required.
Another method is to perform the background detection/suppression after the document processing system input is converted to luminance/chrominance color space such as CIE L*a*b*, where L* represents the dark to light lightness dimension or luminosity, and a*, b* represent the chrominance components which define the difference between a color and a chosen reference color of the same luminous intensity. Typically, the colored document is scanned by a scanner using a set of red, green, and blue sensors which produces a data representation in terms of the RGB signals at each pixel location. The data is then converted into a luminance/chrominance color space.
Typically, the background suppression is performed in the luminance channel. Due to the distribution of the chrominance channel values in the background region, suppressing the luminance channel alone does not suppress background completely. In order to fully suppress the background, the chrominance channels are adjusted. However, the chrominance channels are adjusted independently of each other's chrominance information and also independently of the luminance channel information. This leads to a color shift in some regions.
There is a need for methods and apparatuses that overcome the aforementioned problems and others.